Sintered silicon nitride, components made therewith, specially valves, methods for the production and use thereof

ABSTRACT

A sintered Si 3 N 4  material, valves and components made with the material, and methods for making same.

This application is a 371 of PCT/EP98/01816 filed Mar. 27, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to sintered silicon nitride (Si₃N₄),components made thereof, in particular valves, process for theirproduction and their use.

Materials made of Si₃N₄ are of proven use in many applications. However,components made of Si₃N₄ still have inadequacies such as lack orreliability in continuous use, which stand in the way of wide use ofSi₃N₄ components in the said applications, which would be advantageouseconomically and ecologically. For example, although DE-A 4312251 claimsa high-strength Si₃N₄ material having a defined failure probability, noteaching is given regarding the way in which these failure probabilitiesare to be achieved. They are merely derived from classical flexuralstrength determinations and statistical evaluation thereof.

The reliability of ceramic materials is determined from short-termstrength, with its spread, and from long-term behaviour under load. Inthis context, the short-term strength follows the Griffith relationship:$\begin{matrix}{\sigma = \frac{K_{Ic}}{\sqrt{c}Y}} & (1)\end{matrix}$

with:

σ Strength in MPa

K_(lc) Fracture toughness in MPa·m^(½)

c Critical crack length in μm and

Y Form factor, which describes the shape of the critical crack.

According to this relationship, the strength is directly dependent onthe crack or defect length in the material.

The scatter in the short-term strength, which is important forreliability, is described by the Weibull distribution and ischaracterized by the Weibull modulus according to DIN 51110.

In the case of loading below the stress which leads to catastrophicfracture, however, the “v-k” concept is applicable

v=A·K ^(n) _(l)  (2)

with

v Crack growth rate in m/s,

K_(l) Stress intensity factor in MPa·m^(½) in the case of load type l=tensile stress, and

A,n Parameters for subcritical crack growth (life).

This concept is applicable to materials and components which are exposedto varying stresses below the maximum stress, described by the Griffithrelationship, which immediately leads to failure, and is thereforerelevant to ceramic materials and components for a large number oftechnical applications, e.g. for valves in reciprocating piston engines.

The crack growth parameter is determined, according to the descriptionof standard draft ENV 843-3 by determining flexural strengths atdifferent loading rates.

The determination of the flexural strength is described in DIN 51 110.The loading rate employed in this case, which is intended to causefracture in 5 to 10 s, is customarily about 100 MPa/s. The flexuralstrength determined in this case is referred to as short-term or inertstrength σ_(c).

In order to ascertain the crack growth parameter, this measurement iscarried out at reduced loading rates. In this case, the cracks which arepresent have the opportunity to grow, with the result that fractureoccurs under lower loads, i.e. so-called subcritical crack growth takesplace. If the breaking stress is plotted against the loading rate on adouble logarithmic scale, and the median values of the measurementcarried out repeatedly for a defined loading rate are joined by a bestfit line, then the crack growth parameters n and A are found from theslope of the line and the axis intercept of this line. Typical ceramicmaterials have n values of 30 to 40 (see Kingery, Introduction toCeramics, John Wiley & Sons, New York, 1976, page 804) and are thereforeapparently to be qualified as subcritical crack growth, so that theirlife in practical use is limited.

In order to satisfy increasing demands, especially in the automobileindustry, a need has arisen for Si₃N₄ materials and components withimproved reliability.

The object of the present invention was therefore to provide sinteredSi₃N₄ and reliable components, in particular valves based on Si₃N₄,which have properties meeting this profile and are also straightforwardand therefore inexpensive to produce.

It has unexpectedly been found that sintered Si₃N₄ with a particularchlorine content has improved subcritical crack growth behaviour withhigh flexural strength and high Weibull modulus at the same time.

DESCRIPTION OF THE INVENTION

The invention therefore relates to sintered Si₃N₄ which has a chlorinecontent of 100 to 500 ppm, a subcritical crack growth parameter n≧50,preferably ≧60, a mean flexural strength at room temperature ≧850 MPaand a Weibull modulus ≧18.

The chlorine content of the sintered Si₃N₄ was in this case determinedby pressure digestion with hydrofluoric acid at temperatures between 100and 120° C. and subsequent potentiometric titration of the chloride bymeans of silver nitrate.

The sintered Si₃N₄ according to the invention preferably containsalkaline earth metals, Sc₂O₃, Y₂O₃, rare earth oxides, TiO₂, ZrO₂, HfO₂,B₂O₃ and/or A1₂O₃ as sintering additives, these forming a secondaryphase concentration in the sintered material of 7.5 to 20 vol. % inaddition to crystalline Si₃N₄ and/or Si₃N₄ mixed crystals.

This secondary phase concentration is determined by ascertaining thetotal oxygen content of the sintered Si₃N₄ through hot extraction. Theknown oxygen concentration introduced by the added sintering aids issubtracted from this result. The difference represents the oxygencontent of Si₃N₄ following preparation, which is assumed to be presentin the form of SiO₂. This SiO₂ concentration is added to the sinteringaid concentration, which represents the total proportion of oxideconstituents in addition to Si₃N₄.

For the Si₃N₄ proportion in the material, its pure density of 3.18 g/cm³is employed to calculate the volume fraction, and for the secondaryphases which are formed by the reaction of the sintering additives withthe SiO₂ in the Si₃N₄ powder during the sintering, the pure densityρ_(R) is calculated according to $\begin{matrix}{\rho_{R} = {G\text{-}{{tot}/{\sum\limits_{i = 1}^{i = n}{\left( {G_{i}/\rho_{Ri}} \right)\quad {in}\quad {g/{cm}^{3}}}}}}} & (3)\end{matrix}$

with

G-tot=Total weight of the oxide components in g

G_(i)=Weights of the individual oxide components in g

ρ_(Ri)=Pure densities of the individual oxide components in g/cm³.

The volume fractions of Si₃N₄ and secondary phase are therebydetermined, the latter being between 7.5 and 20 vol. % for the materialaccording to the invention.

The Si₃N₄ according to the invention is distinguished by a high packingfactor (low porosity) so that, for example, during re-sintering at atemperature up to 50° C. higher than the sintering temperature, neitherthe density nor the Young's modulus of the material changes.

The invention also relates to a process for preparing the sintered Si₃N₄according to the invention where Si₃N₄ powder, which either containschlorine in an amount of 500 to 1500 ppm or, as an alternative to this,is used together with a metal chloride, is dispersed in water togetherwith at lest one sintering additive, mixed with organic processing aids,

the aqueous slip is ground to a fineness of 90%<1 μm,

and subsequently dried preferably by spray drying or fluidized beddrying so that the Si₃N₄ granules have a moisture content of between 1.0and 4% by weight, preferably between 1 and 3% by weight and an averagegranule size of 40 to 80 μm, and these are subsequently compressed andsintering is carried out after the organic process aids have been bakedout under an N₂ pressure of 1≦p≦10 bar.

The compression is preferably carried out axially and/or isostatically.

In a preferred embodiment of the invention, the compression is carriedout at pressures <2500 bar, the organic process aids and the moistureare baked out in air, inert gas or vacuum at T≦650° C. and the sinteringis carried out under an N₂ pressure of 1≦p≦10 bar at T≦2000° C.

Preferably, the Si₃N₄ powder used has a Cl content of 500 to 1500 ppmand leads in the sintered Si₃N₄ to a Cl content of 100 to 500 ppm.

Preferred sintering additives which can be used in the process accordingto the invention are alkaline earth metals, Sc₂O₃, Y₂O₃, rare earthoxides, TiO₂, Zro₂, HfO₂, B₂O₃ and/or Al₂O₃.

These are preferably added in amounts such that, during the sintering,by reaction with the oxygen which is always present in Si₃N₄ powders,and is assumed to be in the form of SiO₂, a liquid phase is formed whichis present in the sintered material as a predominantly vitreoussecondary phase in a concentration of 7.5-20 vol. %.

Preferred organic process aids which can be used in the processaccording to the invention are dispersing agents and/or impression aids,the latter comprising the function of binding and plasticizing.

The dispersion agents are preferably citric and polyacrylic acidderivatives and amino alcohols in concentrations of 0.1-2.5% by weightin relation to the solids content of the slip.

A large number of substances can be used as compression aids, such as,for example, polyurethane dispersions, cellulose derivatives, starchesand polysaccharides, polyamide solutions, polyvinyl alcohol and acetate,polyethylene glycols and/or stearates. These are preferably used inamounts of 0.2-5% by weight.

The compression at <2500 bar may be carried out by axial and/orisostatic dry compression, for example in a mould corresponding to thecomponent.

The materials obtained in this way have no granule residues in thematerial structure. Reproducible strengths in excess of 850 MPa withWeibull moduli ≧18 are therefore obtained.

The invention also relates to components, especially valves, made of theSi₃N₄ according to the invention.

The present invention furthermore also relates to valves with a failureprobability of less than 10⁻⁶ made of the sintered Si₃N₄ according tothe invention.

The invention also relates to a process for producing the valvesaccording to the invention, according to which the valves are selectedusing vibration analysis by frequency splitting of the resonantfrequency peak ≧0.0125%. The other process steps are similar to theprocess according to the invention for the preparation of Si₃N₄, thecompression being carried out in a mould corresponding to the valve tobe produced.

It has now been found that, with the last-mentioned process,irrespective of the geometrical shape of the valve, defective componentscan be rapidly and definitively detected through frequency splitting ofresonant frequency peaks.

This kind of selection is, however, also possible for components with adifferent geometrical shape.

In the process according to the invention, the valve to be tested isplaced on the head side on three electrodynamic transducers, onetransmitter and two receivers, in such a way that the valve issupported, at a distance of about 1-2 mm from the edge of the head, withan angular separation of the sensors equal to 120° each. By varying thefrequency of the transmitter in the 0.1 to 2 MHz range, flexuralvibrations of the shaft and flexural vibrations of the head plate arestimulated in the ceramic valve, and are recorded using the twovibration receivers.

Macroscopic defects, such as cracks, inclusions of components extraneousto the material or nonuniformities in terms of the thickness and/orYoung's modulus are manifested by a clearly attributable resonancefrequency split into 2 subsidiary natural vibrations, the frequencysplit increasing as the size of the fault increases.

The invention also relates to the use of the Si₃N₄ according to theinvention and components produced therefrom in engine construction,especially as valves in reciprocating piston engines, in mechanicalengineering in general, storage technology and in machine construction.

Engine parts produced therefrom are distinguished primarily by long lifeand high reliability. For example, Si₃N₄ engine valves on test rigswhich were operated far beyond the conditions encountered in normalreciprocating piston engines, showed outstanding strength withoutfailing.

The invention is further described in the following illustrativeexamples in which all parts and percentages are by weight unlessotherwise indicated.

EXAMPLES

A variety of material compositions were produced in the same way byusing different Si₃N₄ starting powders and sintering additives, and werecharacterized. The substantially chlorine-free LC12S (H.C. Starck,Germany) and the chlorine-containing Baysinid®ST (BAYER AG, Germany)were used as Si₃N₄ powders. These products have the following typicalproperties:

LC12S Baysinid ®ST Specific surface, m²/g 20 >10 O content, % by weight2.0 1.2-1.4 C content, % by weight 0.2 <0.05 Fe content ppm: 60 <50 Alcontent ppm: 300 <20 Ca content ppm: 30 <20 Cl content pm: <100 700-1500 α-Si₃N₄ fraction, % approx. 95 approx. 96 relative to α and β

The following were used as sintering additives: Y₂O₃ (obtainable underthe trade name Grade C from the company H.C. Starck, Germany) and Al₂O₃(obtainable under the trade name CT 3000 SG from the company Alcoa,Germany) and MgO (obtainable as MgO p.a. from the company Merck,Germany). The raw materials were placed in the H₂O p.a. already present,to which a dispersion agent (KV 5088 from the company Zschimmer &Schwarz, Germany) was added, and were dispersed using high-speedstirrers. The viscosity was adjusted to 20 mPas, which corresponded to asolids content around 64%.

The batches were ground in a mechanically agitated ball mill until thesimultaneously measured particle size distribution gave a grading of90%<1 μm.

1% of Bayceram®4305, available from Bayer AG, Germany, was then added asbinder and plasticizer, and grinding was continued for 5 minutes.

Slips prepared in this way were granulated using spray dryers, theconditions having been adjusted in each case in relation to the batch sothat the proportion of granules <150 μm was as low as possible.Granules >150 μm were separated from the batch to be processed using ascreen.

Shaped articles moulded from these granules at <2500 bar were heated totemperatures of up to at most 650° C. and were sintered as in DE-A 4 233602 without embedding in inert powder in graphite containers at 1800°C.±50° C. in an N₂ atmosphere.

The material properties are reported in the table below. The flexuralstrength and the Weibull modulus were determined according to DIN 51 110and the subcritical crack growth parameters were determined according toEuropean draft standard ENV 843-3.

The following abbreviations have been used in the table below:

“TGV-Ans.” corresponds to the proportion of powder particles in thesuspension after grinding which are smaller than 1 μm (%<1 μm). “GGV”the granule size distribution characterized by the mean of thedistribution D50 in μm. The sintered density is the density of thesintered article, determined using the buoyancy method in H₂O, the “Ocontent” and “Cl content” the oxygen (O) and chlorine (Cl) contentsanalysed after sintering in the sintered article. From the O content,after subtracting the known oxygen levels which are introduced by thesintering additives, the oxygen level which was introduced through theSi₃N₄ powder after the preparation step was calculated. With theassumption that this is present in the form of SiO₂, the totalconcentration of the oxide components in % by weight was calculated byadding this calculated SiO₂ content in the sintered material and theconcentration of the sintering additives added. This total concentrationis converted according to the explanations above into volume fractionsof secondary phase and crystalline Si₃N₄. The important secondary phaseconcentration in volume % determined in this way is given in the column“Sec.-pha.” in the table.

TABLE 2 Material variations for Example 1 and resulting materialproperties Sintered articles TGV- GGV Sintering cond. Sintered O contentCl Flexural According to Batch Batch composition Ans. % D-50 T, ° C./t,density % by Sec.-pha. content strength Weibull- the invention No. % byweight <1 μm μm h/p-N2, bar g/cm³ weight Vol. % ppm MPa modulus n valueyes/no 0 LCl2s + 5Y₂O₃ + 91.0 75 1800/2/100 3.23 5.7 14.2 70 895 22 27no 5Al₂O₃ 1 BST + 5Y₂O₃ + 92.0 75 1775/2/10 3.24 5.4 12.9 390 920 22 60yes 5Al₂O₃ + 1MgO 2 BST + 8Y₂O₃ + 91.5 80 1825/2.5/10 3.28 3.8 13.9 320942 28 71 yes 4Al₂O₃ BST is Baysinid ® ST

The effect according to the invention of Cl contents on the subcriticalcrack growth parameter n can be seen clearly from these examples

Although the present invention has been described in detail withreference to certain preferred versions thereof, other variations arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the versions contained therein.

What is claimed is:
 1. A sintered Si₃N₄ consisting essentially of achlorine content of 100 to 500 ppm, wherein the sintered Si₃N₄ materialhas a subcritical crack growth parameter in that is greater than orequal to 50, a mean flexural strength at room temperature that isgreater than or equal to 850 MPa and a Weibull modulus that is greaterthan or equal to
 18. 2. The sintered Si₃N₄ material according to claim1, wherein the material further comprises a sintering additive selectedfrom the group consisting of, alkaline earth metals, Sc₂O₃, Y₂O₃, rareearth oxides, TiO₂, ZrO₂, HfO₂, B₂O₃ and Al₂O₃, wherein the sinteringadditive component forms a secondary phase concentration in the sinteredSi₃N₄ material of 7.5 to 20 vol. % in addition to a component selectedfrom the group consisting of crystalline Si₃N₄ and Si₃N₄ mixed crystals.3. A process for preparing the sintered Si₃N₄ material consistingessentially of a chlorine content of 100 to 500 ppm, wherein thesintered Si₃N₄ material has a subcritical crack growth parameter n thatis greater than or equal to 50, a mean flexural strength at roomtemperature that is greater than or equal to 850 MPa and a Weibullmodulus that is greater than or equal to 18, the process comprising (A)dispersing Si₃N₄ powder in water together with at least one sinteringadditive component, and mixing with the dispersed powder, organicprocessing aids to form an aqueous slip, wherein the Si₃N₄ powder has achlorine content in an amount that ranges from 500 to 1500 ppm, andoptionally a metal chloride; (B) grinding the aqueous slip to a finenessin which 90% of the slip is less than 1 μm, (C) subsequently drying theaqueous slip so that Si₃N₄ granules have a moisture content of between 1and 4% by weight and an average granule size that ranges from 40 to 80μm, (D) subsequently compressing and sintering Si₃N₄ granules after theorganic processing aids have been baked under an N₂ pressure that rangesfrom 1 to 10 bar.
 4. The process according to claim 3, wherein thecompression is carried out at a pressure that is less than or equal to2500 bar, the organic processing aids and the moisture are baked in air,inert gas or vacuum at a temperature that is less than or equal to 650°and the sintering is carried out at a temperature that is less than orequal to 2000° C.